The tissue around a typical cancer tumor is characterized by angiogenesis, which is a growth of nearby blood vessels supplying the growing tumor. Most likely, the tumor contains a lot more blood than healthy tissue. A widely used method of detection is to inject contrast agents and acquire images, either using x-ray imaging or magnetic resonance imaging (MRI). For x-ray imaging, the most widely used contrast agents are based on iodine, which attenuate x-rays more than normal tissue. High concentrations of a contrast agent make the tumor appear like a cloud in x-ray images, but unfortunately the cloud is super-imposed with anatomical structures, which may be strong enough to obscure the tumor. In addition, the amount of injected contrast agents should be kept at moderate levels, since they may cause allergic reactions and kidney failure. According to prior art, image subtraction is a widely used method to remove super-imposed tissue from the images, and obtain a resultant image where only the contrast agent appears. One method is temporal subtraction, where images are acquired before and after injection of contrast agents, and pre-contrast images are subtracted from post-contrast images, whereby all super-imposed structure is removed and only contrast agents remain. One problem of temporal subtraction is motion artifacts, which prevents the method from being used in cardiac imaging, and even breast imaging.
Another method is energy subtraction, where two or more images are acquired using different x-ray spectrum. Typically two images are acquired, based on spectral energies of low and high photon energies. Proper subtraction of high and low energy images cancels out most human tissue, and iodine contrast remains. The method is made possible thanks to the attenuation of iodine that has substantially different spectral curve, related to the electron energies in the K-edge of the iodine atom. The simplest method of achieving energy subtraction is to acquire two images in rapid succession, and in between change the voltage of the x-ray tube.
EP00270761A2 discloses a multi-layer flat-panel detector where photons are absorbed at different depth, depending on energy. The object is to perform energy subtraction, using a single exposure. There are, however, reasons to believe that a cost-effective production method still remains to be invented.
FIGS. 1 and 2 show prior art of a multi-slit x-ray scanner for acquisition of conventional x-ray mammograms without dual energy. The patient is irradiated by a bundle of thin, x-ray beams, each of which is detected by a corresponding line detector. Each beam has a rectangular cross-section, typically 4 centimeters wide and 50 micrometers across. The narrow beams are created by letting the x-rays pass through a collimator 120, which is a metal plate with several narrow linear apertures, referred to as slits. For each slit, there is one corresponding line detector. FIG. 3 illustrates a schematic line detector according to prior art. For visibility, only a few channels are drawn. Each line detector comprises a silicon array of photon conversion channel elements 210 and a corresponding array of pulse-counting circuits 230. Each photon conversion channel element has a width equal to the pixel size, which is 50 micrometers. Each photon conversion channel element converts photons to electric pulses, which are counted in a corresponding pulse counter. There is a strip on the surface of each photon conversion channel element. A strip is a layer on top of the semiconductor, which is connected by a small bond thread to an integrated circuit, comprising pre-amplifiers and pulse counters. The line detectors, the collimator and the x-ray source form an entity that is moveable by a scan motion. During irradiation by the x-ray source, the line detectors are moved relative the image area, whereby the set of line detectors acquire a set of overlapping part images. A resultant image is computed as the average of all overlapping part images. In FIGS. 1-2, the line detectors are mounted in a detector assembly 150. The breast to be irradiated is compressed using a compression plate 140. Among the advantages of multi-slit scanning are outstanding scatter rejection, low dose and photon counting.
WO04091405A1 discloses a multi-slit scanner where collimator slits are equipped with individual moveable filters. Thus, one exposure is enough to irradiate the imaged object with beams of different spectra, and perform dual energy examination. U.S. Ser. No. 05/665,969 discloses an x-ray detector and an x-ray apparatus, where photon energy is measured, and data is weighted depending on importance. Low energetic photons are given higher weight than high energetic photons, since low energy photons are more important. Each photon is absorbed in photon conversion channel element, where it is converted to an electric pulse and the photon energy is measured from the pulse strength. Several solutions are revealed, in particular a system with a limited number of energy levels and photons being accumulated using a weighted sum. There are individual weights at each level of energy. Neither negative weights nor energy subtraction is mentioned.
A spectral analyzing detector for computed tomography is disclosed in “Photon-Counting Detectors for Digital Radiography and X-Ray Computed Tomography”, by Paul C. Johns, Jacques Dubeau, David G. Gobbi, Mei Li, and Madhu S. Dixit, Optoelectronics, Photonics, and Imaging, SPIE TDO1, pages 367-369 (2002). The disclosed detector is a photon-counting gas micro-strip detector for computed tomography (CT), which measures the energy of each individual photon, based on the amount of electrical charge released when each photon hits the gas. Several applications are mentioned, for example corrections for beam hardening in artifacts in CT. Another mentioned application is image subtraction across K-edge energy level, to enhance iodine-based contrast agents.